1. Field of the Invention
The present invention relates to a plastic optical waveguide used in applications, such as optical communication and the like, and an optical switch using the same.
2. Description of the Related Art
There have been rapid advances in technical development directed to popularization of general public as subscribers, such as private homes, of optical communication networks for realization of an a concept referred to as FTTH (Fiber To The Home). In regard to an optical node unit intended for use by a subscriber, high performance, high reliability and a lower cost have become the most important objectives in such development.
In parallel to such a movement, a study concerning the application of optical communication technology to a computer has simultaneously progressed, and in such an application, higher integration and hybridization of electronic elements have been called for.
As a technology to meet such needs, developments concerning an optical waveguide and an optical fiber made of an high polymer material have energetically been conducted. The physical properties required for such a material are (1) low propagation loss, (2) low birefringence and (3) high temperature endurance.
In such circumstances, the earliest material developed was a polymethacrylic acid resin, but though it has excellent optical properties, either the high temperature endurance or resistance to humidity properties were found to be not sufficient. Then polystyrene, polycarbonate and the like were studied, but the glass transition temperatures of these materials are on the order of 150.degree. C., and thereby they exhibit a low temperature endurance, and the inherent birefringence is about 20 times as high as that of acrylic resin.
Very recently, fluorine containing polyimide, which has an improved high temperature endurance, has been proposed (see Publication of Unexamined Japanese Patent Application No. Hei 3-72528), which material is an optical plastic satisfying a low loss requirement in an infrared region.
However, the polyimide has a large birefringence caused by its properties of orientation and anisotropy in polarization. Therefore, when a waveguide is constructed with the material, there arises an intrinsic birefringence in the range of 0.1 to 0.3, which has been a serious problem.
In the case where an optical waveguide or an optical fiber having such a birefringence is incorporated in an optical network, a great limitation arises in the total length of an element because of so-called mode dispersion. That is, taking a network having a capacity of 10 Gbit/s as an example, an allowable length of an element is 300 mm, and, as a result, it is impossible to construct an optical network system having a sufficient scale.
TABLE 1 ______________________________________ birefringence allowable length of element ______________________________________ 0.1 30 mm 0.01 300 mm 0.001 3,000 mm ______________________________________
In the Publication of Unexamined Japanese Patent Application No. Hei 7-56030, there is disclosed an optical waveguide using polyimide, including an alicyclic diamine as an diamine component, as a plastic material which has improved on that problem.
There is, however, still a problem that a loss in the 1.3 gm band is as large as, for example, 0.8 dB/cm.
Polyimide is obtained by applying a heat treatment and the like to a coat of a polyamic acid solution, which is generally obtained in a reaction between an acid dianhydride and a diamine to cause a dehydration ring-closing reaction.
In the case where an aromatic acid dianhydride as the acid dianhydride component and an aliphatic amine as the diamine component are used, it is known that, since an acidity of polyamic acid produced and a basicity of a diamine produced are both high, a carboxyl group of polyamic acid and amine forms a strong salt. For this reason, it is considered that an amine group is left behind in the polyimide, so that a N--H stretching vibration in the 1.3 .mu.m band arises, which in turn causes a large optical loss.